Lithofacies, diagenesis and reservoir quality of Upper Shu’aiba reservoirs in northwestern Oman

Abstract

ABSTRACT Core and log data from Lower Cretaceous limestones of the Upper Shu’aiba Member were used to characterize the distribution of lithofacies, clay and porosity within two low-angle clinoforms that form the reservoirs for an oilfield of northwestern Oman. Data from 15 vertical wells, including four with core, and four horizontal well cores were projected into a dip-oriented cross-section derived from a static reservoir model as a basis for visualizing the above variations. Each clinoform consists of a basal “argillaceous zone” and a thicker “reservoir zone” of clean limestone, together reflecting fourth-order cycles of progradation along the margin of the Bab intra-shelf basin. Lithofacies vary in a proximal direction from mudstone and wackestone (mid-ramp) to mud-dominated packstone (slope) to mud-rich floatstone, rudstone and boundstone (ramp crest) and are arranged in a pattern of decreasing water depth and increasing energy both upwards and landward within each clinoform. In contrast, the reservoir zone of a younger clinoform from a nearby oilfield consists of well-sorted grainstone and grain-dominated packstone, illustrating the wide range of depositional conditions that occurred in the ramp-crest facies belt of different units. Except within the proximal extent of the younger clinoform, where values are transitional toward reservoir zone values, the argillaceous zones have total porosity mostly < 10% and baseline-normalized gamma-ray (GR) activity > 23 API units, reflecting clay contents of around 10–18%. In contrast, most parts of the reservoir zones have GR of 15–23 API units and porosity of 10–35%. Higher clay content is suggested to be linked with lower porosity through facilitation of both mechanical and chemical compaction, the latter providing a local supply of calcite cement. XRD analyses show that the clays are kaolin, illite/smectite and illite, similar to the clays in the overlying Nahr Umr shale. Most former macropores have been filled by blocky calcite cement in the main oilfield studied, but all lithofacies have similar wide ranges of total porosity of 8% to > 30%. The cores were also studied for evidence of diagenesis related to the contact with the overlying Nahr Umr Formation, but profiles of stable-isotope ratios, bulk-rock strontium, petrography and porosity-permeability data show no trends indicative of upward-increasing meteoric diagenesis below this sequence boundary. Meteoric leaching could nevertheless be pervasive throughout the Upper Shu’aiba reservoirs, at least partially accounting for extensive aragonite dissolution and low Sr and δ18O values. Two of the cores show trends of upwards-increasing bulk-rock uranium, manganese and iron, possibly indicative of sea-floor authigenesis. In addition, saddle dolomite near the tops of these cores may reflect late influx of magnesium derived from clay in the Nahr Umr Formation.